Conveying Fresh Pet Food After the Chiller: Moisture, Tray Weight and the Four-Degree Line

At 06:40 the blast chiller door opens on 240 trays of gently cooked chicken and rice, core temperature 3 °C, and they come out the way chilled trays do — dull with condensation, film pulled tight and fogged, a thin layer of liquid in the tray well where the gravy has settled. The operator stacks them six high and starts date-coding. Ninety seconds later the top tray is still 3 °C inside. Its film is no longer fogged. It is wet, and the label goes on over water.

That ninety seconds is the whole subject. Nothing about fresh pet food is difficult to cook. What is difficult is that the product leaves the chiller at the edge of its allowed temperature and spends the rest of its short life being moved through rooms that are warmer than it is.

1. A Moment at the Chiller Door

2. What Fresh Pet Food Is, Physically

3. Where Conveying Sits on This Line

4. Three Places It Goes Wrong

5. Why This Belt Face and Not Another

6. The Numbers That Set the Design

7. Changeover and Cleaning

8. The Sentence That Decides It

The tray is 2.4 kg, roughly 220 × 160 mm, and not rigid — a filled tray flexes at the corners when picked up by one end. Six stacked trays weigh close to 15 kg, and the bottom one carries all of it through a film lid.

The room the trays are stacked in runs at 10–12 °C, about 8 °C warmer than the product. At that difference, and at the humidity a washdown hall carries, the film drops below the dew point within seconds of leaving the chiller. Condensation is not a defect in the sealing machine. It is the room's answer to a cold surface, and it arrives on every tray, all shift.

Gently cooked fresh pet food — the chilled, short-life category rather than the retort or dried ones — is a complete diet cooked at 70–90 °C, filled warm into trays or pouches, then chilled hard and fast. What matters to a conveyor:

· Moisture, 60–78%. The product is mostly water held in a protein and starch matrix, and the water leaves the matrix under any mechanical pressure.

· Structure, soft and irregular. Chunks of 8–20 mm in a gravy or a loose mince. Nothing in the tray resists a point load; the tray resists even less.

· Mass, 0.4–2.5 kg per unit, in trays, pouches or shallow tubs. The packaging is the only thing holding the shape.

· Temperature, 0–4 °C after chilling, with a surface that is wet by definition.

· Shelf life, single-digit to low-double-digit days, carried entirely by the cold chain. There is no preservative load to absorb a warm afternoon.

· Wet and mildly greasy. Gravy is sticky when it dries and slippery when it does not.

The cooking end is pumps, pipes and a filler. The conveying problem starts after the seal and runs to the cold store door.

Six metres sounds short for a conveyor section. On this product it is the most consequential six metres on the line, because every one is spent outside the chiller.

Water on the belt becomes a slide. The film on a chilled tray carries a continuous water layer within a minute of leaving the chiller. On a smooth belt that layer behaves like a lubricant, and friction at the tray-to-belt interface drops by roughly half. Nothing dramatic happens on the level: trays drift a few millimetres, invisible until the code sits 4 mm off centre on every tray. On the case-pack incline it stops being cosmetic: a 2.4 kg tray that slips 20 mm as the case is pushed rotates, the film corner lifts, and the case reaches the cold store with a lid that is no longer sealed.

Gravy leaves the tray and stays in the machine. Tray wells collect liquid during chilling, and a filled tray crossing a transfer plate at any speed leaves a trail. It runs down the frame, into the return way and onto the wear strips, mixing with belt dust into a film that sets hard. In a chilled room the film does not dry, so it stays biologically active between washes, and the smell that greets the morning shift comes from the return side of the belt, not from the product.

The temperature budget is spent on the belt. A tray leaves the chiller at 3 °C and has to enter the cold store at 4 °C or below. Ninety seconds on an open belt in a 12 °C hall lifts the surface layer of a shallow tray by two or three degrees, and the product is not uniform — the gravy at the edges warms first. The visible consequence is condensation and a label lifting at the corner. The invisible one is a temperature record that no longer matches the shelf life printed on the tray.

Two surfaces are in common use here, and they fail in opposite directions.

A flat PU belt presents a continuous, smooth, easily cleaned surface. It is the cheaper of the two per metre, it is quiet, and the top face wipes down in a few minutes. What it does not do is drain. Water and gravy stay on the carrying surface until they leave at the discharge, and a tray riding a water film has no traction.

A modular plastic slat chain — the open-hinge type used across filled-container lines — drains through the hinge gaps and carries the liquid away from the product rather than under it. The tray sits on discrete surfaces instead of a film. The cost is real: more edges to clean, more noise, a higher purchase price, and hinge gaps that hold product if the section is not opened regularly.

A Mesh Belt Conveyor solves the drainage problem even more completely and is worth considering where the section is straight and the tray footprint is large enough not to catch on the mesh. It brings its own cleaning burden, since everything that passes the mesh lands on whatever is underneath.

The choice comes down to how much water the trays shed.

Six figures change the layout, all measurable before anything is quoted:

· Trays per minute, average and peak, against the case packer's rate — the gap is what the section absorbs.

· Unit mass and footprint, plus the stack height used in the cold store. Six trays of 2.4 kg is a different conveyor from one of 0.4 kg.

· Chiller exit core temperature, measured at the centre rather than the surface. The gap between the two is what the belt fights.

· Minutes allowed between chiller exit and case pack — a quality parameter that belongs in the layout.

· Room temperature and relative humidity, and whether the hall is washed down during production.

· Washdown chemistry and frequency — alkaline, chlorinated or plain water, and how often the section is opened.

The fourth one is usually missing from the enquiry, and it is the one that decides whether the section is four metres or nine.

The section is cleaned daily, sometimes twice, and the sequence matters more than the chemicals.

Dry removal first: trays out, film scraps off, standing liquid scraped into a container rather than pushed toward the drain. Then detergent at the discharge end, worked back against the direction of product travel, so loosened material does not cross a cleaned surface; rinse water follows the same direction. The belt is opened at the discharge, the hinge gaps brushed, and the return way flushed separately, because it is the part that holds the smell.

Twenty-five to forty minutes for four metres, most of it on the return side and the transfer plates.

The four-degree line on a fresh pet food plant is kept by the first ninety seconds after the chiller door, not by the chiller — and those ninety seconds are decided by a belt face, a drainage path and a length of conveyor that usually appears on the layout as a single line.

Orange Zhou, Technical Sales Manager · orange@xstlconveyor.com